Ultrasonic motor

ABSTRACT

Provided herein is an ultrasonic motor in which at least fluorocarbon polymer is present on the contact surface of at least either of the rotor or the stator. Since the fluorocarbon polymer provides a smooth contact surface, the ultrasonic motor generates no noise and produces a stable starting torque and holding torque. In addition, it has a long life with a minimum of wear.

This is a continuation application of Ser. No. 07/477,198, filed Feb. 6,1990, now U.S. Pat. No. 5,150,00, issued Sep. 22, 1992, which in turn isa continuation-in-part of application Ser. No. 07/259,703, filed Oct.19, 1988, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to an ultrasonic motor which is sodesigned as to produce a driving force by the aid of traveling waves ofultrasonic frequency vibration generated by piezoelectric elements.

An ultrasonic motor driven by traveling waves, as disclosed in, forexample, U.S. Pat. No. 4,562,373, is made up of a flat piezoelectricelement, a flat stator to which is attached said piezoelectric element,a rotor which is in contact under pressure with said stator, and afriction material interposed between said stator and said rotor. To runthe motor, an electric power of ultrasonic frequency is applied to thepiezoelectric element, with the time difference and phase differenceproperly controlled. The applied electric power generates transversetraveling waves in synchronism with the ultrasonic vibration in thethickness direction of the piezoelectric element and stator. The wavecrests of the traveling waves drive the rotor forward through thefriction material.

The ultrasonic motor of traveling wave type will be described withreference to FIG. 5. There are shown a piezoelectric element 1, a stator2 which is bonded and fixed to the piezoelectric element 1, a rotor 3placed on the stator 2, and a friction material 4 firmly attached to therotor 3. When energized, the piezoelectric element 1 generates travelingwaves of ultrasonic frequency which propagate in the direction of arrowA in the stator 2. The vibration causes each particle of the stator 2 tomove along the elliptic orbit indicated by arrow B. The wave crest ofthe traveling wave advances horizontally in the direction opposite tothat of the traveling wave, and the wave trough of the traveling waveadvances horizontally in the same direction as the traveling wave.Therefore, the rotor 3 placed on the stator 2 is in contact with thewave crests only and is driven horizontally in the direction of arrow Cby frictional force generated between the rotor 3 and the stator 2.

An important factor in the ultrasonic motor of such structure is how therotor is in contact under pressure with the stator. It greatly affectsthe starting torque, no-load speed, motor-efficiency, and motor life.Heretofore, much has been studied on increasing friction between therotor and the stator. To this end, a variety of materials have beeninvestigated. For example, there was proposed a method of increasingfriction by attaching a metallic material or rubber having a highcoefficient of friction to the rotor and bringing the rotor into contactwith the stator under pressure. However, attaching a metallic materialto the rotor needs precision working for the contact surfaces of themetallic material and the stator. An additional disadvantage is that themetal-to-metal contact produces a noise which is a problem in practicaluse.

A friction material other than metallic ones is a rubber slider whichhas a high coefficient of friction. The commercial one, however, has adisadvantage. That is, it is liable to wear as the contact surface isrubbed. The wearing adversely affects the starting torque, no-loadspeed, and motor efficiency, and eventually shortens the motor life. Inaddition, the rubber slider does not keep the holding torque (or brakingtorque) constant. (Holding torque is a force to hold the rotor inposition when the motor is at rest.)

SUMMARY OF THE INVENTION

The present invention was completed to solve the above-mentionedproblems involved in the prior art. Accordingly, it is an object of thepresent invention to provide an ultrasonic motor which meets thefollowing requirements for practical use. (1) The noise level is as lowas possible when the motor is running. (2) The drive force (torque)produced by contact under pressure should be as possible. (3) The wearof the contact surface should be as small as possible so that the motorkeeps its performance over a long period of time. (4) The holding torqueto keep the rotor in position when the motor is at rest should beconstant. The gist of the present invention resides in an ultrasonicmotor having a platy stator provided with a platy piezoelectric elementand a rotor which is in contact under pressure with said stator suchthat said rotor is driven through a friction means attached thereto bythe wave crests of the transverse traveling wave of ultrasonic frequencywhich is generated on the surface of the stator in the thicknessdirection of said piezoelectric element and said stator when an electricpower of ultrasonic frequency is applied to said piezoelectric element,characterized in that at least a fluorocarbon polymer is present on thecontact surface of at least either said rotor or said stator.

While the novel features of the invention are set forth in the appendedclaims, the invention, both as to organization and content will bebetter understood and appreciated, along with other objects and featuresthereof, from the following detailed description taken in conjunctionwith the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the basic structure of anultrasonic motor;

FIG. 2 is a perspective view showing the structure of a discoidultrasonic motor;

FIG. 3 is a perspective view showing the structure of a toroidalultrasonic motor;

FIG. 4 is an enlarged view of powder particles having a plurality ofwhisker-like projections, said powder particles being an elementconstituting the present invention; and

FIG. 5 is a schematic representation to explain the principle on whichthe ultrasonic motor works.

DETAILED DESCRIPTION OF THE INVENTION

The ultrasonic motor of the present invention has the basic structure asshown in FIG. 1. It is made up of a platy piezoelectric element 1, aplaty stator 2 to which is attached said piezoelectric element 1, arotor 3 which is in contact under pressure with said stator 2, and atleast a fluorocarbon polymer on the contact surface of at least eithersaid stator 2 or said rotor 3.

The stator in contact with the rotor is usually made of a metallicmaterial such as steel and stainless steel which has a low dampingfactor for vibration. Ultrasonic motors embodied according to thepresent invention include discoid one and toroidal one as shown in FIGS.1 and 2, respectively.

The embodiments of the present invention are described in the followingin which the friction material is at least a fluorocarbon polymer.

(a) Embodiment in Which the Friction Material is a Plastic MaterialContaining Fluorocarbon Resin Powder Example 1

The friction material 4a shown in FIG. 2 was prepared in the followingmanner. At first, 5 parts by weight of polytetrafluoroethylene powder(having an average particle diameter of 10 μm) was mixed weight (assolids) of resin BT-2121, which is a bismaleimide-triazine resincontaining 30 wt % of flexibilizer, to effect uniform dispersion. Theresulting mixture was molded into a 0.2 mm thick sheet by compressionmolding at 200° C. under 100 kg/cm² for 2 hours. (Thepolytetrafluoroethylene powder should preferably be of lubricant gradehaving an average particle diameter smaller than 20 μm.)

The thus obtained friction material 4a was bonded to the rotor 3a, andthe contact surface of the friction material was polished. The resultingrotor assembly was mated with the stator. Thus there was obtained adiscoid ultrasonic motor, 40 mm in diameter, as shown in FIG. 2. Therotor was pressed against the stator under a pressure of 4 kg/cm²applied by a spring. The ultrasonic motor produced a starting torque of800 gf·cm and a no-load speed of 1100 rpm with an input power of about 8W.

To see how the ultrasonic motor changes with time in performance, it wasturned one million times continuously at 500 rpm under a load of 300gf·cm. During this test run, the ultrasonic motor remained almostunchanged in performance (such as speed, torque, and efficiency). Afterthe test run, no change was noticed on the contact surface of thestator. The wear of the contact surface of the plastics frictionmaterial measured after the test run was 10 μm. This wear is muchsmaller than that in the case of metal-to-metal friction. The ultrasonicmotor in this example did not produce noise at all.

For the purpose of comparison, the same ultrasonic motor as mentionedabove was prepared, except that the friction material was made of aresin. The motor was run under the same conditions as mentioned above tosee how it changes with time in performance. The motor graduallydecreased in speed and came to a halt after about 20,000 turns.Presumably, this happened because there is nothing to lubricate thestator and rotor.

Example 2

The friction material 4a as shown in FIG. 2 was prepared in thefollowing manner. At first, 4 parts by weight of polytetrafluoroethylenepowder (having an average particle diameter of 5 μm) was mixed with 5parts by weight of phenolic resin and 1 part by weight of aromaticpolyamide fiber cut to 2 mm (in the form of pulp) using an intensivemixer for 1 hour at room temperature. This mixing causes thepolytetrafluoroethylene powder and aromatic polyamide fiber to uniformlydisperse into the matrix resin. The resulting mixture was molded into a0.2 mm thick sheet by compression molding at 150° C. under 100 kg/cm²for 2 hours. Also, different kinds of friction materials were preparedin the same manner as mentioned above, except that the aromaticpolyamide fiber was replaced by other fibers. The thus obtainedsheet-like friction materials were evaluated in the same manner as inExample 1. The results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Plastics components     Motor performance                                                      Polytetra-                                                                           Starting                                                                           No-load                                                           fluoroeth-                                                                           torque                                                                             speed                                                                              Wear                                        Kind of fiber                                                                              wt %                                                                              ylene (wt %)                                                                         (gf · cm)                                                                 (rpm)                                                                              (μm)                                     __________________________________________________________________________    Aromatic polyamide fiber                                                                   10  40     650  1100  8                                          Carbon fiber 10  40     700  1150  7                                          Glass fiber  15  35     650  1100  9                                          Silicon carbide fiber                                                                       5  45     650  1250 12                                          Alumina fiber                                                                               5  45     700  1300 14                                          Quartz fiber  5  45     650  1250 12                                          Asbestos     15  35     700  1100 11                                          __________________________________________________________________________

Although thermosetting polyimide resin and phenolic resin were used asthe matrix resin in the above-mentioned Examples 1 and 2, it is alsopossible to use other thermosetting resins and thermoplastic resins. Inaddition, it is also possible to control the friction performance byincorporating the plastics matrix with iron powder, copper powder, zincpowder, tin powder, stainless steel powder, powder of alloy of saidmetals, graphite powder, aluminum powder, molybdenum sulfide powder,mica powder, silicon carbide powder, or fluorocarbon powder.

(b) Embodiment in Which the Friction Material is Composed of a MatrixResin and at Least Fluorocarbon Resin Powder and Carbon Fiber Example 3

Five kinds of friction materials (A to E) in the form of 1 mm thicksheet were prepared by combining 4 kinds of matrix resins, 5 kinds offluorocarbon resin powders, and 6 kinds of carbon fibers as shown inTable 2. For comparison, two kinds of friction materials (F and G)containing no fluorocarbon resin powder were prepared. The thus preparedfriction material was bonded to a stainless steel disc, 40 mm indiameter and 1 mm thick, to make a test piece.

The test piece was tested for dynamic friction which was produced whenthe test piece was turned at 30 rpm, with a stainless steel ball, 3 mmin diameter, in contact under a load of 200 g with the test piece at theposition 15 mm apart from the center. The dynamic friction which changedwith time was recorded. The results are shown in Table 3.

                                      TABLE 2                                     __________________________________________________________________________    Desig-                                                                            Carbon    Fluorocarbon Matrix                                             nation                                                                            fiber wt %                                                                              resin powder                                                                           wt %                                                                              resin wt %                                         __________________________________________________________________________    A   PAN-based                                                                           20  Tetrafluoro-                                                                           15  Polyimide                                                                           65                                               short fiber                                                                             ethylene polymer,                                                             4 μm in dia.                                                 B   PAN-based                                                                           10  Tetrafluoro-                                                                           25  Polyimide                                                                           65                                               pulp-like ethylene polymer,                                                   fiber     2 μm in dia.                                                 C   Pitch-based                                                                         30  Fluororubber                                                                           10  Polyimide                                                                           60                                               short fiber            (BT resin)                                         D   Pitch-based                                                                         20  Tetrafluoro-                                                                           15  Phenolic                                                                            65                                               pulp-like ethylene polymer                                                                           resin                                                  fiber                                                                     E   Phenolic-                                                                           15  Hexafluoro-                                                                            15  Rubber-                                                                             70                                               based     propylene    modified                                               short fiber                                                                             polymer      phenolic                                                                      resin                                               F* PAN-based                                                                           20  --       --  Polyimide                                                                           80                                               short fiber                                                                G* Pulp-like                                                                           30  --       --  Phenolic                                                                            70                                               asbestos               resin                                                  fiber                                                                     __________________________________________________________________________     *Comparative Examples                                                    

                  TABLE 3                                                         ______________________________________                                        Coefficient of Dynamic Friction                                                       Designation of Friction Materials                                     Time      A       B      C    D    E    F    G                                ______________________________________                                        Initial   0.23    0.21   0.29 0.23 0.24 0.28 0.33                             After 10 min.                                                                           0.24    0.22   0.30 0.25 0.25 0.37 0.44                             After 20 min.                                                                           0.25    0.22   0.31 0.25 0.26 0.44 0.54                             After 30 min.                                                                           0.25    0.22   0.31 0.25 0.26 0.49 0.61                             After 60 min.                                                                           0.25    0.22   0.31 0.25 0.26 0.51 0.66                             After 90 min.                                                                           0.25    0.22   0.31 0.25 0.26 0.51 0.68                             After 120 min.                                                                          0.25    0.25   0.31 0.25 0.26 0.51 0.68                             ______________________________________                                    

It is noted from Tables 2 and 3 that the composite friction materialscomposed of a matrix resin, fluorocarbon resin powder, and carbon fiberhave a high coefficient of friction (higher than 0.2) which changed verylittle with time. By contrast, the friction materials (F and G inComparative Examples) which contain no fluorocarbon resin powder greatlychanged with time in coefficient of friction.

Example 4

Seven discoid ultrasonic motors as shown in FIG. 2 were prepared inwhich the friction material 4a is one of those which are designated as Ato G in Example 3. The ultrasonic motor is made up of a piezoelectricelement 1a, a stator 2a of stainless steel which is bonded to thesurface of the piezoelectric element 1a, a rotor 4 of stainless steel,and a friction material 4a attached to the rotor 3a. The frictionmaterial 4a is made of the composite plastic sheet prepared in Example3. The rotor so that an initial braking torque of 500 gf·cm is produced.The piezoelectric element is provided with four electrodes which arearranged so that four traveling waves are generated in thecircumferential direction of the stator. The rotor 3a was driven at ano-load speed of 500 rpm by applying an electric field of resonancefrequency, about 70 kHz. After running for a prescribed period of time,the motors were tested for restarting performance, braking torque (whichis produced after de-energizing), and resonance frequency. The resultsare shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________                  Designation of Friction Materials                               Items         A   B   C   D   E   F   G                                       __________________________________________________________________________    Braking torque (gf · cm)                                             Initial       500 500 500 500 500  500                                                                               500                                    After 1 hour  510 510 520 510 520 1000                                                                               900                                    After 2 hours 510 510 530 520 530 1400                                                                              1300                                    After 3 hours 510 500 530 520 530 1400                                                                              1600                                    After 5 hours 510 500 530 520 530 1400                                                                              1600                                    Restarting performance                                                        Initial       Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                    After 5 hours Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Poor                                                                              Poor                                    Resonance frequency (kHz)                                                     Initial       71.0                                                                              71.2                                                                              72.0                                                                              71.5                                                                              72.5                                                                              72.0                                                                              73.0                                    After 5 hours 71.3                                                                              71.3                                                                              72.5                                                                              71.9                                                                              72.9                                                                              74.0                                                                              75.5                                    Wear of stator surface                                                        After 5 hours Small                                                                             Small                                                                             Small                                                                             Small                                                                             Small                                                                             Great                                                                             Great                                   Wear of friction material                                                     After 5 hours Small                                                                             Small                                                                             Small                                                                             Small                                                                             Small                                                                             Small                                                                             Great                                   Squeak during running                                                                       None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              Yes Yes                                     __________________________________________________________________________

It is noted from Table 4 that the ultrasonic motors equipped with one ofthe friction materials A to E, which are composed of a matrix resin,fluorocarbon resin powder, and carbon fiber, changed with time verylittle in braking torque and resonance frequency. It is also noted thatthey had no problem with restarting and the stator 2a of stainless steelwore very little. By contrast, the ultrasonic motors equipped with oneof the friction materials F and G, which do not contain fluorocarbonresin powder, greatly changed with time in braking torque and resonancefrequency. In addition, they had a problem with restarting.

Example 5

Five kinds of composite plastics friction materials (designated as I toM) were prepared from polyimide (as a matrix resin), tetrafluoroethyleneresin powder having an average particle diameter of 3 μm (as afluorocarbon resin powder), and PAN-based pulp-like carbon fiberaccording to the formulation shown in Table 5. Each friction materialwas formed into a 1-mm thick sheet. This sheet was attached to the samediscoid ultrasonic motors as used in Example 2. The ultrasonic motorswere driven by applying an electric field of resonance frequency in thesame manner as in Example 2. After running for a prescribed period oftime, the motors were tested for restarting performance, braking torque(which is produced after de-energizing), and resonance frequency. Theresults are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                                  Designation of Friction Materials                                   Items       I        J       K     L     M                                    ______________________________________                                        PAN-based pulp-                                                                             5       10      10    5     30                                  like carbon fiber                                                             (wt %)                                                                        Tetrafluoroethylene                                                                       --         2      5     5     20                                  resin powder                                                                  (wt %)                                                                        Polyimide (wt %)                                                                           95       88      85    90    50                                  Braking torque                                                                (gf · cm )                                                           Initial      500      500    500   500   500                                  After 1 hour                                                                              1000      900    550   530   510                                  After 2 hours                                                                             1300     1000    560   540   520                                  After 3 hours                                                                             1300     1000    560   540   520                                  After 5 hours                                                                             1300     1000    560   540   520                                  Restarting                                                                    performance                                                                   Initial     Good     Good    Good  Good  Good                                 After 5 hours                                                                             Poor     Poor    Good  Good  Good                                 Resonance                                                                     frequency (kHz)                                                               Initial     70.2     71.6    71.4  71.3  71.3                                 After 5 hours                                                                             74.3     73.3    71.7  71.7  71.4                                 Wear of stator                                                                surface                                                                       After 5 hours                                                                             Great    Great   Small Small Small                                Wear of friction                                                              material                                                                      After 5 hours                                                                             Medium   Small   Small Small Small                                Squeak during                                                                             Yes      Yes     None  None  None                                 driving                                                                       ______________________________________                                    

It is noted from Table 5 that the ultrasonic motors provided with one ofthe friction materials K, L, and M containing 5 wt % or more offluorocarbon resin powder and 5 wt % or more of carbon fiber changedwith time very little in braking torque and resonance frequency and hadno problem with restarting. By contrast, the ultrasonic motors providedwith one of the friction materials I and J containing 2 wt % or less offluorocarbon resin powder greatly changed with time in braking torqueand resonance frequency and were poor in restarting performance.

Example 6

A composite plastic friction material was prepared by impregnating feltwith a phenolic resin solution in which are uniformly dispersed carbonfiber and fluorocarbon resin powder. After preliminary drying, thefriction material was compression molded into a 1-mm thick sheet. Thissheet was used as the friction material 4a in the discoid ultrasonicmotor as shown in FIG. 2.

Example 7

A composite plastic friction material was prepared by impregnatinglaminated plain weaves of carbon fiber with a polyimide resin solutionin which are uniformly dispersed fluorocarbon resin fine powder havingan average particle diameter of 1 μm. After preliminary drying, thefriction material was compression molded into a 1-mm thick sheet. Thissheet was used as the friction material 4a in the discoid ultrasonicmotor as shown in FIG. 2.

The ultrasonic motors in both Examples 6 and 7 had an initial brakingtorque of 500 gf·cm, which slightly changed to 530 gf·cm after runningfor 5 hours. They had resonance frequencies of 70.8 kHz and 70.6 kHz,respectively, which remained almost unchanged even after running for 5hours. They had no problem with restarting. The friction material didnot wear nor scratch the stator 2a of stainless steel with which it isin contact.

(c) Embodiment in Which the Friction Material is Composed of a MatrixResin and at Least Fluorocarbon Resin Powder and Organic Fiber

The matrix resin that can be used in this embodiment includes, forexample, polyimide, polyamideimide, epoxy resin, phenolic resin,silicone resin, polyester resin, polyarylate resin, and liquid-crystalpolymer. The organic fiber that can be used in this embodiment includes,for example, aromatic polyamide fiber, phenolic fiber, organic polymerwhisker, polybenzimidazole fiber, supermolecular-weight polyethylenefiber, and liquid-crystal plastic fiber. The amount of the organic fiberand fluorocarbon resin powder should be 5 wt % or more each. Thefriction material used in this embodiment may be further incorporatedwith an inorganic or metallic filler.

Example 8

Five kinds of friction materials (A1 to E1) in form of 1 mm thick sheetwere prepared from a composite material composed of a matrix resin andfluorocarbon resin powder and organic fiber as shown in Table 6. Forcomparison, two kinds of friction materials (F1 and G1) containing nofluorocarbon resin powder were prepared as shown in Table 6. The thusprepared friction material was bonded to a stainless steel disc, 40 mmin diameter and 1 mm thick, to make a test piece.

The test piece was tested for dynamic friction which was produced whenthe test piece was turned at 30 rpm, with a stainless steel ball (3 mmin diameter) in contact under a load of 200 g with the test piece at theposition 15 mm apart from the center. The dynamic friction which changedwith time was recorded. The results are shown in Table 7.

                                      TABLE 6                                     __________________________________________________________________________    Desig-                                                                             Organic   Fluorocarbon                                                                             Matrix                                              nation                                                                             fiber wt %                                                                              resin powder                                                                         wt %                                                                              resin wt %                                          __________________________________________________________________________    A1   Pulp-like                                                                           10  Tetrafluoro-                                                                         25  Polyimide                                                                           65                                                 aromatic  ethylene                                                            polyamide polymer                                                             "Kevlar"                                                                 B1   Pulp-like                                                                           15  Tetrafluoro-                                                                         20  Phenolic                                                                            65                                                 aromatic  ethylene   resin                                                    polyamide polymer                                                             "Cornex"                                                                 C1   Phenolic                                                                            20  Hexafluoro-                                                                          20  Polyimide                                                                           60                                                 polymer   propylene  "BT resin"                                               short fiber                                                                             polymer                                                        D1   Aromatic                                                                            20  Fluororubber                                                                         25  Polyamide-                                                                          55                                                 polyamide            imide                                                    short fiber                                                                   "HM-50"                                                                  E1   Polyoxy-                                                                            15  Tetrafluoro-                                                                         20  Liquid                                                                              65                                                 methylene ethylene   crystal                                                  whisker   polymer    polyester                                            F1* Pulp-like                                                                           15  --     --  Polyimide                                                                           85                                                 aromatic                                                                      polyamide                                                                     "Kevlar"                                                                  G1* Pulp-like                                                                           30  --     --  Phenolic                                                                            70                                                 asbestos             resin                                               __________________________________________________________________________     *Comparative Examples                                                    

                  TABLE 7                                                         ______________________________________                                        Coefficient of Dynamic Friction                                                       Designation of Friction Materials                                     Time      A1      B1     C1   D1   E1   F1   G1                               ______________________________________                                        Initial   0.23    0.21   0.26 0.28 0.25 0.29 0.33                             After 10 min.                                                                           0.25    0.23   0.28 0.30 0.26 0.38 0.44                             After 20 min.                                                                           0.25    0.23   0.29 0.31 0.26 0.45 0.54                             After 30 min.                                                                           0.25    0.23   0.29 0.32 0.26 0.51 0.61                             After 60 min.                                                                           0.25    0.23   0.29 0.32 0.26 0.53 0.66                             After 90 min.                                                                           0.25    0.23   0.29 0.32 0.26 0.53 0.68                             After 120 min.                                                                          0.25    0.23   0.29 0.32 0.26 0.53 0.68                             ______________________________________                                    

It is noted from Tables 6 and 7 that the composite friction materialscomposed of a matrix resin, fluorocarbon resin powder, and organic fiberhave a high coefficient of friction (higher than 0.2) which changed withtime very little. By contrast, the friction materials (F1 and G1 inComparative Examples) which contain no fluorocarbon resin powder greatlychanged with time in coefficient of friction.

Example 9

Seven discoid ultrasonic motors as shown in FIG. 2 were prepared inwhich the friction material 4a is one of those which are designated asA1 to G1 in Example 8. The ultrasonic motor is made up of apiezoelectric element 1a, a stator 2a of stainless steel which is bondedto the surface of the piezoelectric element 1a, a rotor 4a of stainlesssteel, and a friction material 4a attached to the rotor 3a. The frictionmaterial 4a is made of the composite plastic sheet prepared in Example8. The rotor 3a is pressed against the stator 2a by means of a spring sothat an initial braking torque of 500 gf·cm is produced. Thepiezoelectric element is provided with four electrodes which arearranged so that four traveling waves are generated in thecircumferential direction of the stator. The rotor 3a was driven at ano-load speed of 500 rpm by applying an electric field of resonancefrequency, about 70 kHz. After running for a prescribed period of time,the motors were tested for restarting performance, braking torque, andresonance frequency. The results are shown in Table 8.

                                      TABLE 8                                     __________________________________________________________________________                  Designation of Friction Materials                               Items         A1  B1  C1  D1  E1  F1  G1                                      __________________________________________________________________________    Braking torque (gf · cm)                                             Initial       500 500 500 500 500  500                                                                               500                                    After 1 hour  520 510 520 520 500 1100                                                                               900                                    After 2 hours 520 520 520 530 510 1500                                                                              1300                                    After 3 hours 520 520 520 540 510 1500                                                                              1600                                    After 5 hours 520 520 520 540 510 1500                                                                              1600                                    Restarting performance                                                        Initial       Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                    After 5 hours Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Poor                                                                              Poor                                    Resonance frequency (kHz)                                                     Initial       71.5                                                                              71.5                                                                              71.3                                                                              71.8                                                                              71.3                                                                              72.5                                                                              73.0                                    After 5 hours 71.9                                                                              71.8                                                                              71.6                                                                              72.6                                                                              71.5                                                                              74.5                                                                              75.5                                    Wear of stator surface                                                        After 5 hours Small                                                                             Small                                                                             Small                                                                             Small                                                                             Small                                                                             Great                                                                             Great                                   Wear of friction material                                                     After 5 hours Small                                                                             Small                                                                             Small                                                                             Small                                                                             Small                                                                             Small                                                                             Great                                   Squeak during running                                                                       None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              Yes Yes                                     __________________________________________________________________________

It is noted from Table 8 that the ultrasonic motors equipped with one ofthe friction materials A1 to E1, which are composed of a matrix resin,fluorocarbon resin powder, and organic fiber, changed with time verylittle in braking torque and resonance frequency. It is also noted thatthey had no problem with restarting and the stator 2a of stainless steelwore very little. By contrast, the ultrasonic motors equipped with oneof the friction materials F1 and G1, which do not contain fluorocarbonresin powder, greatly changed with time in braking torque and resonancefrequency. In addition, they had problems with restarting.

Example 10

Five kinds of composite plastics friction materials (designated as I1 toM1) were prepared from polyimide, fluorocarbon resin powder, and organicfiber according to the formulation shown in Table 9. Each frictionmaterial was formed into a 1-mm thick sheet. This sheet was attached tothe same discoid ultrasonic motors Example 9. The ultrasonic motors weredriven by applying an electric field of resonance frequency in the samemanner as in Example 9. After running for a prescribed period of time,the motors were tested for restarting performance, braking torque, andresonance frequency. The results are shown in Table 9.

                  TABLE 9                                                         ______________________________________                                                  Designation of Friction Materials                                   Items       I1       J1      K1    L1    M1                                   ______________________________________                                        Aromatic Polyamide                                                                          5       10      10    5     20                                  fiber (wt %)                                                                  Tetrafluoroethylene                                                                       --         2      5     5     20                                  resin powder                                                                  (wt %)                                                                        Polyimide (wt %)                                                                           95       88      85    70    60                                  Braking torque                                                                (gf · cm )                                                           Initial      500      500    500   500   500                                  After 1 hour                                                                              1000      800    550   530   520                                  After 2 hours                                                                             1400     1000    560   540   520                                  After 3 hours                                                                             1400     1000    560   540   520                                  After 5 hours                                                                             1400     1000    560   540   520                                  Restarting                                                                    performance                                                                   Initial     Good     Good    Good  Good  Good                                 After 5 hours                                                                             Poor     Poor    Good  Good  Good                                 Resonance                                                                     frequency (kHz)                                                               Initial     72.3     71.8    71.6  71.5  71.5                                 After 5 hours                                                                             74.5     73.6    72.0  71.9  71.7                                 Wear of stator                                                                surface                                                                       After 5 hours                                                                             Great    Great   Small Small Small                                Wear of friction                                                              material                                                                      After 5 hours                                                                             Medium   Small   Small Small Small                                Squeak during                                                                             Yes      Yes     None  None  None                                 driving                                                                       ______________________________________                                    

It is noted from Table 9 that the ultrasonic motors provided with one ofthe friction materials K1, L1, and M1containing 5 wt % or more offluorocarbon resin powder and 5 wt % or more of organic fiber changedwith time very little in braking torque and resonance frequency and hadno problems with restarting. By contrast, the ultrasonic motors providedwith one of the friction materials I1 and J1 containing 2 wt % or lessof fluorocarbon resin powder greatly changed with time in braking torqueand resonance frequency and were poor in restarting performance.

(d) Embodiment in Which the Friction Material is Made of FluorocarbonResin Containing at Least Carbon Fiber or Carbon Powder

The carbon fiber that can be used in this embodiment is not specificallylimited; it may be in the form of short fiber, long fiber, continuousfilament, woven cloth, felt, or paper. The carbon powder that can beused in this embodiment is not specifically limited either. It may be inthe form of graphite or carbon. In this embodiment, the following sixfriction materials, A2 to F2, were prepared for evaluation.

(1) Friction Material A2

A woven cloth of carbon fiber.sup.(1) was impregnated with an aqueousdispersion of tetrafluoroethylene resin.sup.(2). After drying, 10 sheetsof the impregnated cloths placed on top of the other were sintered at370° C. under a pressure of 300 kg/cm². The resulting molded sheetcomposed of 65% of carbon fiber and 35% of tetrafluoroethylene resin wasground to a thickness of 0.5 mm.

(2) Friction Material B2

A felt of carbon fiber.sup.(1) was impregnated with an aqueousdispersion of tetrafluoroethylene resin.sup.(2). After drying, 10 sheetsof the impregnated felts placed on top of the other were sintered at370° C. under a pressure of 300 kg/cm². The resulting molded sheetcomposed of 55% of carbon fiber and 45% of tetrafluoroethylene resin wasground to a thickness of 1 mm.

(3) Friction Material C2

Chopped carbon fiber.sup.(1) was mixed with tetrafluoroethylene resinpowder.sup.(2) using a mixer. The mixture was sintered in a mold at 370°C. under a pressure of 400 kg/cm². The resulting molded article composedof 25% of carbon fiber and 75% of tetrafluoroethylene resin was groundto a thickness of 1 mm.

(4) Friction Material D2

Graphite powder.sup.(1) was mixed with tetrafluoroethylene resinpowder.sup.(2) using a mixer. The mixture was sintered in a mold at 370°C. under a pressure of 400 kg/cm². The resulting molded article composedof 35% of graphite powder and 65% of tetrafluoroethylene resin wasground to a thickness of 2 mm.

(6) Friction Material E2

A woven cloth of carbon fiber.sup.(1) was impregnated with an aqueousdispersion of tetrafluoroethylene resin.sup.(2). After drying, theimpregnated cloth was rolled into a cylinder, followed by sintering at380° C. in an autoclave under a pressure of 100 kg/cm². The resultingcylindrical molded article composed of 65% of carbon fiber and 35% oftetrafluoroethylene resin was cut in round slices, 3 mm thick.

(6) Friction Material F2

For the purpose of comparison, a 1 mm thick friction layer was made bygrinding tetrafluoroethylene resin (formed by sintering under pressure"Polyflon M-15", made by Daikin Kogyo Co., Ltd.).

Six discoid ultrasonic motors were prepared in which the frictionmaterial is one of those (designated as A2 to F2) prepared as mentionedabove. The piezoelectric element is provided with four electrodes whichare arranged so that four traveling waves are generated in thecircumferential direction of the stator. The rotor was driven byapplying an electric power of 80 V at a resonance frequency of 70 kHz.After running for a prescribed period of time, the motors were testedfor restarting performance, braking torque after de-energizing, wear ofthe stator surface, wear of the friction material, and squeaking duringrunning. The results are shown in Table 10.

                                      TABLE 10                                    __________________________________________________________________________                 Designation of Friction Materials                                Items        A2  B2  C2  D2  E2  F2                                           __________________________________________________________________________    Holding torque (gf · cm)                                             Initial      1500                                                                              1500                                                                              1500                                                                              1500                                                                              1500                                                                              300                                          After 30 minutes                                                                           1530                                                                              1540                                                                              1550                                                                              1560                                                                              1520                                                                              360                                          After 1 hour 1550                                                                              1570                                                                              1590                                                                              1600                                                                              1520                                                                              410                                          After 2 hours                                                                              1550                                                                              1570                                                                              1600                                                                              1610                                                                              1520                                                                              430                                          After 24 hours                                                                             1550                                                                              1570                                                                              1600                                                                              1610                                                                              1520                                                                              430                                          Restarting performance                                                        Initial      Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                         After 24 hours                                                                             Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Poor                                         Squeaking during running                                                      Initial      None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              Yes                                          After 24 hours                                                                             None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              Yes                                          Wear of stator surface                                                        After 24 hours                                                                             None                                                                              Small                                                                             Small                                                                             None                                                                              None                                                                              None                                         Wear of friction material                                                     After 24 hours                                                                             <1 μm                                                                          <1 μm                                                                          <1 μm                                                                          1 μm                                                                           <1 μm                                                                          70 μm                                     __________________________________________________________________________

It is noted from Table 10 that the ultrasonic motors equipped with oneof the friction materials A2 to E2 changed with time very little inholding torque and had no problems with restarting and noise. It is alsonoted that the friction material and stator wore very little aftercontinuous running for 24 hours. By contrast, the ultrasonic motorequipped with the friction material F2, which is made of fluorocarbonresin alone, did not start when the screw was tightened so that aninitial holding torque of 1500 gf·cm is produced. It started when thescrew was loosened until the initial holding torque decreased to 300gf·cm. However, it did not restart smoothly after continuous running for24 hours. Moreover, it generated noise during running. The frictionmaterial wore as much as 70 μm after continuous running for 24 hours.Incidentally, the low holding torque leads to the low starting torque.Then, the following three friction materials G2, H2, and I2 wereprepared, and their performance was evaluated in the same manner asmentioned above.

(7) Friction Material G2

A felt of carbon fiber.sup.(1) was impregnated with an aqueousdispersion of trifluorochloroethylene resin.sup.(2). After drying, theimpregnated felt was sintered at 310° C. under a pressure of 300 kg/cm².The resulting molded sheet composed of 55% of carbon fiber and 45% oftrifluorochloroethylene resin was ground to a thickness of 1 mm.

(8) Friction Material H2

A woven cloth of carbon fiber.sup.(1) was impregnated with an aqueousdispersion of tetrafluoroethylene-hexafluoropropylene copolymerresin.sup.(2). After drying, 10 sheets of the impregnated cloths placedon top of the other were sintered at 370° C. under a pressure of 300kg/cm². The resulting molded sheet composed of 65% of carbon fiber and35% of tetrafluoroethylene-hexafluoropropylene copolymer resin wasground to a thickness of 1 mm.

Tetrafluoroethylene resin powder.sup.(1) was mixed with 25% of graphitepowder.sup.(2) and 15% of silicon carbide whisker powder.sup.(3) using amixer. The mixture was sintered in a mold at 370° C. under a pressure of400 kg/cm². The resulting molded article was ground to a thickness of 2mm.

Three discoid ultrasonic motors were prepared in which the frictionmaterial is one of those (designated as G2, H2, and I2) prepared asmentioned above. After running for prescribed period of time, the motorswere tested for restarting (which occurs when the powder source isturned on and off intermittently), holding torque after de-energizing,wear of the stator surface, wear of the friction material, and squeakingduring running. The results are shown in Table 11.

                  TABLE 11                                                        ______________________________________                                                       Designation of Friction Materials                              Items            G2        H2      I2                                         ______________________________________                                        Holding torque (gf · cm)                                             Initial          1500      1500    1500                                       After 30 minutes 1560      1550    1570                                       After 1 hour     1600      1590    1610                                       After 2 hours    1600      1590    1620                                       After 24 hours   1600      1590    1620                                       Restarting performance                                                        Initial          Good      Good    Good                                       After 24 hours   Good      Good    Good                                       Squeaking during running                                                      Initial          None      None    None                                       After 24 hours   None      None    None                                       Wear of stator surface                                                        After 24 hours   Small     None    Small                                      Wear of friction material                                                     After 24 hours   <1 μm  <1 μm                                                                              <1 μm                                   ______________________________________                                    

It is noted from Table 11 that the ultrasonic motors equipped with oneof the friction materials G2, H2, and I2 changed with time very littlein holding torque and had no problems with restarting and noise. It isalso noted that the friction material and stator wore very little aftercontinuous running for 24 hours.

The above-mentioned embodiments show that the friction material composedof fluorocarbon resin containing carbon fiber or carbon powder wears outvery little and prevents the stator from wearing, and consequently theultrasonic motor provided with it changes with time very little inholding torque, exhibits good restarting performance, and produces nonoise.

Incidentally, in the above-mentioned embodiments, the fluorocarbon resinmay be incorporated with other fibrous or powdery fillers in addition tothe carbon fiber and carbon powder.

(d) Embodiment in Which the Friction Material is a Molded ArticleContaining 50 to 95 wt % of Fluorocarbon Polymer Example 11

A friction material was prepared by mixing (in dry process) 7 parts byweight of polytetrafluoroethylene powder (as fluorocarbon polymer) and 3parts by weight of liquid-crystal all-aromatic polyester resin powder

The effect attained by the carbon fiber or carbon powder in thisembodiment is that it produces very little wear on the vibrating bodysurface even when the highest content of it is employed for suppressingthe wear of friction material at its minimum. Thus, it is possible touse the carbon fiber or carbon powder at its higher amounts, and by thisincreased content, the higher motor pressure can be attained. As shownin this embodiment, stable motor operating characteristics can berealized even when a motor holding torque of 1500 g·cm is realized.

Furthermore, in the case where carbon fiber is employed, the highestamount is 50 weight percent for attaining friction material withadequate mechanical strength and uniform carbon fiber distribution,while an amount of more than 50 weight percent is possible if woven orfelt form carbon fiber is employed. ("Econol", resistant to hightemperatures above 400° C.). The powder mixture was compression-moldedunder 350 kg/cm², followed by sintering at 370° C., to give a 1.0 mmthick sheet. The sheet underwent surface treatment with a commercialsurface treating agent for the improvement of adhesion.

The sheet of friction material was bonded to the rotor with a commercialadhesive (glass fiber-containing epoxy resin type). The surface of thebonded sheet was ground with a cemented carbide cutting tool for theimprovement of surface precision and the removal of the surface treatingagent.

The rotor was mated with a stator, 40 mm in outside diameter, made ofstainless steel, under a pressure of 4 kg/cm² applied by a spring, sothat the rotor has a holding torque of 1200 gf·cm (which is a forcerequired to start the rotor). The resulting ultrasonic motor produced astarting torque of 1000 gf·cm and ran at a no-load speed of 1000 rpmwhen an input power of about 8 W was applied. To see how the ultrasonicmotor changes with time in performance, it was turned two million timescontinuously at a speed of 500 rpm under a load of 500 gf·cm. Theultrasonic motor remained almost unchanged in torque, efficiency, andholding torque. In addition, the stator surface in contact with thefriction material remained unchanged. The contact surface of thefriction material wore only 4 μm after continuous running for 200 hours.This amount of wear is by far smaller than that (100 μm or more) in thecase of metal-to-metal friction under the same condition. The ultrasonicmotor in this example did not produce noise at all unlike the one inwhich metals rub against each other.

For the purpose of comparison, the friction material was prepared frompolytetrafluoroethylene alone. It was not of practical use because ofits poor mechanical strength, low torque, and excessive wear. In otherwords, the effect of the present invention is produced only when thefriction material is incorporated with 5 wt % or more of filler. On theother hand, the friction material cannot be made into a sufficientlystrong sheet if it contains less than 50 wt % ofpolytetrafluoroethylene.

The above-mentioned result conflicts with that in the case of theconventional friction material made of rubber which provides asufficient frictional force. In other words, it was found that despiteits low coefficient of friction, polytetrafluoroethylene under pressureprovides a certain amount of driving force (torque) proportional to thepressure. It was also found that the friction material made ofpolytetrafluoroethylene wears very little, maintains its performanceover a long period of time, and provides a constant holding torque whenthe motor is at rest.

The sheet of the friction material should be thicker than 0.1 mm;otherwise, it absorbs vibrations very little and generates noise as inthe case of metal-to-metal contact. With a thickness greater than 2.0mm, it absorbs transverse traveling waves, decreasing the efficiency, onaccount of its flexibility.

Example 12

A friction material was prepared by mixing (in dry process) 7 parts byweight of polytetrafluoroethylene powder (as fluorocarbon polymer) and 3parts by weight of milled glass fiber. The mixture wascompression-molded under 800 kd/cm², followed by sintering at 370° C.,to give a 0.5 mm thick sheet. The sheet underwent surface treatment witha commercial surface treating agent for the improvement of adhesion. Thefriction material was also incorporated, in addition to glass fiber,with a fibrous filler (such as aromatic polyamide fiber, carbon fiber,silicon carbide fiber, alumina fiber, quartz fiber, and asbestos fiber)or a powdery filler (such as graphite powder, fluorinated graphitepowder, and molybdenum sulfide powder), for the purpose of mechanicalreinforcement, wear reduction, and long-term stability. The ultrasonicmotors provided with the friction material mentioned above gave goodresults.

Examples of the fluorocarbon polymer that can be used in the presentinvention include, in addition to polytetrafluoroethylene,tetrafluoroethylene-hexafluoropropylene copolymer,polychlorotrifluoroethylene,tetrafluoroethylene-perfluoroalkylvinylether copolymer, polyvinylidenefluoride, polyvinyl fluoride, ethylene-tetrafluoroethylene copolymer,and chlorotrifluoroethylene-ethylene copolymer.

(f) Embodiment in Which the Friction Material is a Molded ArticleContaining Uniformly Dispersed-Whisker-Like Powder as a Filler

The whisker-like powder that can be used in this embodiment is notspecifically limited; but it should preferably be a needle-like powderhaving a diameter smaller than 5 μm and an aspect ratio greater than 5,composed mainly of potassium titanate, silicon nitride, or siliconcarbide. The amount of the whisker-like powder in the friction materialshould preferably be 2 to 100 parts by weight for 100 parts by weight ofthe fluorocarbon resin.

The whisker-like powder may be one which has three or more projectionsas shown in FIG. 4. The projections should preferably have a diametergreater than 10 μm and an aspect ratio greater than 2. The amount ofsuch a powder in the friction material should preferably be 2 to 100parts by weight for 100 parts by weight of the fluorocarbon resin.

The friction material filled with uniformly dispersed powder can beproduced by uniformly mixing a fluorocarbon resin powder with awhisker-like powder or a powder having whisker-like projections,compression molding the mixture, and sintering the molded product at amelting point of the fluorocarbon resin.

The friction material can also be incorporated, in addition to theabove-mentioned whisker-like powder or powder having whisker-likeprojections, with molybdenum sulfide powder, carbon fiber, carbonfluoride powder, inorganic powder, organic powder, or metallic powder.

In this embodiment, friction materials designated as A3 to I3 wereprepared in the following manner.

Friction Material A3

90 parts by weight of tetrafluoroethylene resin powder.sup.(1) wasuniformly mixed with 10 parts by weight of silicon carbide whiskerpowder.sup.(2). The mixture was compression molded under 500 kg/cm²,followed by sintering at 360° C. for 2 hours, to give a 1 mm thickmolded product of fluorocarbon resin filled with uniformly dispersedwhisker powder. The resulting molded product was ground to a thicknessof 0.5 mm.

Friction Material B3

85 parts by weight of tetrafluoroethylene resin powder.sup.(1) wasuniformly mixed with 15 parts by weight of silicon nitride whiskerpowder.sup.(2). The mixture was compression molded under 500 kg/cm²,followed by sintering at 360° C. for 2 hours, to give a 2 mm thickmolded product. The resulting molded product was ground to a thicknessof 1 mm.

Friction Material C3

70 parts by weight of tetrafluoroethylene-hexafluoropropylene copolymerresin powder.sup.(1) was uniformly mixed with 30 parts by weight ofpotassium titanate whisker powder.sup.(2). The mixture was compressionmolded at 360° C. under a pressure of 200 kg/cm² to give a 1 mm thickmolded product. The resulting molded product was ground to a thicknessof 0.5 mm.

Friction Material D3

80 parts by weight of tetrafluoroethylene resin powder.sup.(1) wasuniformly mixed with 20 parts by weight of zinc oxide powder having 3 to5 whisker-like projections.sup.(2). The mixture was compression moldedunder 500 kg/cm², followed by sintering at 360° C. for 2 hours, to givea 1.5 mm thick molded product. The resulting molded product was groundto a thickness of 1 mm.

Friction Material E3

75 parts by weight of tetrafluoroethylene resin powder.sup.(1) wasuniformly mixed with 20 parts by weight of zinc oxide powder having 3 to5 whisker-like projections.sup.(2) and 5 parts by weight of molybdenumsulfide powder.sup.(3). The mixture was compression molded under 500kg/cm², followed by sintering at 360° C. for 2 hours, to give a 1.5 mmthick molded product. The resulting molded product was ground to athickness of 1 mm.

Friction Material F3

85 parts by weight of tetrafluoroethylene resin powder.sup.(1) wasuniformly mixed with 10 parts by weight of silicon carbide whiskerpowder.sup.(2) and 3 parts by weight of carbon powder (having a particlediameter smaller than 1 μm). The mixture was compression molded under500 kg/cm², followed by sintering at 360° C. for 2 hours, to give a 1.5mm thick molded product. The resulting molded product was ground to athickness of 1 mm.

Friction Material G3

70 parts by weight of tetrafluoroethylene resin powder.sup.(1) wasuniformly mixed with 25 parts by weight of potassium titanate whiskerpowder.sup.(2) and 5 parts by weight of fluorinated carbonpowder.sup.(3). The mixture was compression molded under 500 kg/cm²,followed by sintering at 360° C. for 2 hours, to give a 1.5 mm thickmolded product. The resulting molded product was ground to a thicknessof 1 mm.

Friction Material H3 (For Comparison)

A 1 mm thick friction material was made from fluorocarbon resin alone.

Friction Material I3 (For Comparison)

A 1 mm thick friction material was made from an engineering plasticcomposed of phenolic resin and asbestos fiber.

The compressive modulus and the coefficient of friction of the frictionmaterials prepared as mentioned above were measured. The results areshown in Table 12. (The coefficient of friction was calculated from thefriction resistance a pressing stylus (3 mm in diameter, made ofstainless steel 303) produces when it is pressed under a load of 200 gagainst the surface of the friction material turning at 500 rpm, withthe contact position being 10 mm away from the center.

Each of the above mentioned friction materials A3 to I3 was used in theultrasonic motor as shown in FIG. 3. The ultrasonic motor is providedwith electrodes so that four traveling waves are generated in thecircumferential direction of the disc. The rotor is pressed against thestator with a proper force which is established by measuring the brakingtorque.

The resonance frequency was measured which changes according as thebraking torque changes in the range of 500 to 1500 gf·cm. The ultrasonicmotor was run with a braking torque of 1000 gf·cm by the application ofelectric power at 80 V and a frequency close to the resonance frequency.The no-load speed and starting torque were measured. Also, after runningfor a prescribed period of time, the braking torque and the wear of thefriction material were measured and the surface of the stator wasexamined for scratches. During running, the squeaking of the motor waschecked. The results are shown in Table 12.

                                      TABLE 12                                    __________________________________________________________________________                   Designation of Friction Materials                              Items          A3  B3  C3  D3  E3  F3  G3  H3  I3                             __________________________________________________________________________    Compressive modulus, kg/mm.sup.2                                                             110 120 150 180 190 120 150 40  700                            Coefficient of friction                                                                      0.26                                                                              0.26                                                                              0.24                                                                              0.27                                                                              0.21                                                                              0.25                                                                              0.22                                                                              0.14                                                                              0.48                           Resonance frequency (kHz) at                                                  different braking torque                                                      at 500 gf · cm                                                                      71.5                                                                              71.6                                                                              72.0                                                                              72.5                                                                              72.7                                                                              71.5                                                                              72.0                                                                              70.5                                                                              75.0                           at 1000 gf · cm                                                                     72.0                                                                              72.2                                                                              72.8                                                                              73.6                                                                              73.8                                                                              72.1                                                                              72.8                                                                              70.8                                                                              78.5                           at 1500 gf · cm                                                                     72.5                                                                              72.7                                                                              73.5                                                                              74.7                                                                              74.6                                                                              72.7                                                                              73.4                                                                              71.2                                                                              81.0                           Braking torque (gf · cm)                                             Initial        1000                                                                              1000                                                                              1000                                                                              1000                                                                              1000                                                                              1000                                                                              1000                                                                              300 1000                           After 30 minutes                                                                             1010                                                                              1010                                                                              1020                                                                              1010                                                                              1010                                                                              1010                                                                              1010                                                                              360 1200                           After 1 hour   1020                                                                              1020                                                                              1030                                                                              1020                                                                              1010                                                                              1010                                                                              1020                                                                              410 1550                           After 2 hours  1030                                                                              1030                                                                              1040                                                                              1030                                                                              1010                                                                              1010                                                                              1020                                                                              430 1650                           After 24 hours 1030                                                                              1030                                                                              1040                                                                              1030                                                                              1010                                                                              1010                                                                              1020                                                                              430 1650                           Starting torque, gf · cm                                                            610 610 550 620 500 570 530 230 800                            Speed of revolution, rpm                                                                     730 740 750 770 780 740 750 700 680                            Restarting performance                                                        Initial        Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Poor                           After 24 hours Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Poor                                                                              Poor                           Squeak during running                                                         Initial        None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              Yes                            After 24 hours None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              Yes Yes                            Wear of stator surface                                                        After 48 hours None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              Great                          Wear of friction material                                                     After 48 hours 4 μm                                                                           4 μm                                                                           6 μm                                                                           2 μm                                                                           <1 μm                                                                          3 μm                                                                           4 μm                                                                           130 μm                                                                         25 μm                       Starting torque gf · cm                                              After 24 hours 600 610 520 620 500 560 520 150 300                            __________________________________________________________________________

It is noted from Table 12 that all the ultrasonic motors provided withone of the friction materials A3 to G3 pertaining to the presentinvention have a resonance frequency which fluctuates only a little(about 1 to 2 kHz) when the pressure is changed (in terms of brakingtorque in the range of 500 to 1500 gf·cm), and also have a brakingtorque which changes with time only a little. The friction materialspertaining to the present invention wear very little and scratch thestator very little. The ultrasonic motors maintained the stableperformance over a long period of time, exhibited the stable restartingperformance, and generated no noise. The starting torque (in the rangeof 530 to 620 gf·cm) remained almost unchanged after continuous runningfor 24 hours. In the above-mentioned experiments, the friction materialsA3, B3, and C3 are made of a fluorocarbon resin filled with whisker-likepowder; the friction material D3 is made of a fluorocarbon resin filledwith powder having whisker-like projections; and the friction materialsE3, F3, and G3 are made of fluorocarbon resin incorporated withmolybdenum sulfide powder, carbon powder, or fluorinated carbon powder.

By contrast, the ultrasonic motor provided with the friction material H3(for comparison) does not start when a pressure to produce a brakingtorque of 1000 gf·cm is applied. It starts when the pressure is reducedso that a braking torque of 300 gf·cm is applied; but the startingtorque is only 230 gf·cm at that time. In addition, the frictionmaterial wore to a great extent and the ultrasonic motor did not restartsmoothly after continuous running for 24 hours.

In the case of the ultrasonic motor provided with the friction materialI3 (for comparison) made of an engineering plastic, the resonancefrequency greatly fluctuated according as the pressure changed and thebraking torque greatly changed with time. The restarting of the motorwas unstable The starting torque decreased to 40% of the initial valueafter continuous running for 24 hours. The motor generated noise.

The effect attainable by the employment of whisker-formed powder in thisembodiment is that the wear of friction material can be minimized whileinflicting very little damage on the vibrating body, and this effect isconsidered to originate from not only the filler material effects, butthe form factor produce by the entanglement between the whiskers orfluorocarbon polymer whiskers themselves.

Furthermore, a substantial improvement of the starting characteristics,which is a problem when fluorocarbon polymer simplex is employed, can beaccomplished by this, and thus a higher motor pressure can be realized.

As shown in this embodiment, stable motor characteristics can beobtained even when a motor braking torque (holding torque) of 1000 g·cmis realized.

(g) Embodiment in Which the Friction Material is a Molded ArticleContaining a Uniformly Dispersed Filler

The filler that can be used in this embodiment is a powder of organicpolymeric material The powder is not specifically limited, and itincludes, for example, polyimide resin-powder, aromatic polyamide resinpowder, aromatic polyester resin powder, benzoguanamine resin powder,polyether ketone resin powder, phenolic resin powder, epoxy resinpowder, and polyester sulfone resin powder. These resin powders maycontain an inorganic filler. Incidentally, the friction material in thisembodiment may contain, in addition to the organic polymeric materialand fluorocarbon resin, other fibrous or powdery fillers. In thisembodiment, friction materials designated as A4 to F4 were prepared inthe following manner.

Friction Material A4

70 parts by weight of tetrafluoroethylene resin powder.sup.(1) wasuniformly mixed with 30 parts by weight of aromatic polyester resinpowder.sup.(2). The mixture was compression molded under 500 kg/cm²,followed by sintering at 360° C. for 2 hours, to give a 1 mm thickmolded product. The resulting molded product was ground to a thicknessof 0.5 mm.

Friction Material B4

50 parts by weight of tetrafluoroethylene resin powder.sup.(1) wasuniformly mixed with 50 parts by weight of polyimide resinpowder.sup.(2). The mixture was compression molded under 500 kg/cm²,followed by sintering at 360° C. for 1 hour, to give a 2 mm thick moldedproduct. The resulting molded product was ground to a thickness of 1.5mm.

Friction Material C4

100 parts by weight of aqueous dispersion of tetrafluoroethyleneresin.sup.(1) was uniformly mixed with 40 parts by weight of aromaticpolyamide resin powder.sup.(2). The mixture was dried in a vacuum at 50°C. and the residual solids were crushed. The resulting powder wascompression molded under 500 kg/cm², followed by sintering at 360° C.for 1 hour, to give a 1 mm thick molded product. The resulting moldedproduct was ground to a thickness of 0.7 mm.

Friction Material D4

75 parts by weight of tetrafluoroethylene resin powder.sup.(1) wasuniformly mixed with 20 parts by weight of benzoguanamine resinpowder.sup.(2) and 5 parts by weight of graphite powder.sup.(3). Themixture was compression molded under 500 kg/cm², followed by sinteringat 360° C. for 1 hour, to give a 1 mm thick molded product. Theresulting molded product was ground to a thickness of 0.3 mm.

Friction Material E4 (For Comparison)

A 1 mm thick friction material was made from fluorocarbon resin alone bycompression molding under 500 cm², followed by sintering at 360° C. for2 hours.

Friction Material F4 (For Comparison)

A 1 mm friction material was made from a composite resin composed ofpolyamideimide resin and inorganic filler.

The compressive modulus and the coefficient of friction of the frictionmaterials prepared as mentioned above were measured. The results areshown in Table 13. The coefficient of friction was calculated from thefriction resistance a pressing stylus (3 mm in diameter, made ofstainless steel 303) produces when it is pressed under a load of 200 gagainst the surface of the friction material turning at 500 rpm, withthe contact position being 10 mm away from the center.

Each of the above mentioned friction materials A4 to E4 was used in theultrasonic motor as shown in FIG. 2. The ultrasonic motor is providedwith electrodes so that four traveling waves are generated in thecircumferential direction of the disc. The rotor is pressed against thestator with a proper force which is established by measuring the brakingtorque.

The resonance frequency was measured which changes according as thebraking torque changes in the range of 500 to 1500 gf·cm. The ultrasonicmotor was run with a braking torque of 1000 gf·cm by the application ofelectric power at 80 V and a frequency close to the resonance frequency.The no-load speed and starting torque were measured. Also, after runningfor a prescribed period of time, the braking torque and the wear of thefriction material were measure and the surface of the stator wasexamined for scratches. During running, the squeaking of the motor waschecked. The results are shown in Table 13.

                                      TABLE 13                                    __________________________________________________________________________                   Designation of Friction Materials                              Items          A4  B4  C4  D4  E4  F4                                         __________________________________________________________________________    Compressive modulus, kg/mm.sup.2                                                             180 150 130 190 40  800                                        Coefficient of friction                                                                      0.31                                                                              0.27                                                                              0.27                                                                              0.25                                                                              0.14                                                                              0.45                                       Resonance frequency (kHz) at                                                  different braking torque                                                      at 500 gf · cm                                                                      72.5                                                                              72.0                                                                              71.8                                                                              72.8                                                                              70.5                                                                              75.0                                       at 1000 gf · cm                                                                     73.7                                                                              72.8                                                                              72.4                                                                              73.9                                                                              70.8                                                                              78.3                                       at 1500 gf · cm                                                                     74.8                                                                              73.5                                                                              72.9                                                                              74.8                                                                              71.2                                                                              81.5                                       Braking torque (gf · cm)                                             Initial        1000                                                                              1000                                                                              1000                                                                              1000                                                                              300 1000                                       After 30 minutes                                                                             1010                                                                              1020                                                                              1010                                                                              1030                                                                              360 1200                                       After 1 hour   1020                                                                              1030                                                                              1020                                                                              1050                                                                              410 1550                                       After 2 hours  1020                                                                              1030                                                                              1020                                                                              1050                                                                              430 1650                                       After 24 hours 1020                                                                              1030                                                                              1020                                                                              1050                                                                              430 1650                                       Starting torque, gf · cm                                                            680 630 620 520 230 750                                        Speed of revolution, rpm                                                                     780 730 750 750 700 680                                        Restarting performance                                                        Initial        Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Poor                                       After 24 hours Good                                                                              Good                                                                              Good                                                                              Good                                                                              Poor                                                                              Poor                                       Squeak during running                                                         Initial        None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              Yes                                        After 24 hours None                                                                              None                                                                              None                                                                              None                                                                              Yes Yes                                        Wear of stator surface                                                        After 24 hours None                                                                              None                                                                              None                                                                              None                                                                              None                                                                              Yes                                        Wear of friction material                                                     After 24 hours 3 μm                                                                           3 μm                                                                           3 μm                                                                           2 μm                                                                           70 μm                                                                          5 μm                                    __________________________________________________________________________

Table 13 shows the following. The friction materials A4 to D4, which aremade of a fluorocarbon resin and a heat-resistant organic polymericmaterial, have a higher coefficient of friction and a greatercompressive modulus (3 times or above) than the friction material (E4)made of a fluorocarbon resin alone. Therefore, the ultrasonic motorsprovided with one of the friction materials A4 to D4 gave a startingtorque of 520 to 680 gf·cm when the rotor is pressed against the statorwith a force which generates a braking torque of 1000 gf·cm. Bycontrast, the ultrasonic motor provided with the friction material E4(for comparison) did not start when a pressure to product a brakingtorque of 1000 gf·cm was applied. It started when the pressure wasreduced so that a braking torque of 300 gf·cm was applied; but thestarting torque was only about 230 gf·cm at that time.

The ultrasonic motors provided with one of the friction materials (A4 toD4) pertaining to the present invention have a resonance frequency whichfluctuates only a little (about 1.5 to 2.5 kHz) when the pressure ischanged (in terms of braking torque in the range of 500 to 1500 gf·cm),and also have a braking torque which changes with time only a little.The friction materials wear very little and scratch the stator verylittle. Therefore, the ultrasonic motors maintained the stable driveperformance regardless of the pressure change and exhibited the stablerestarting performance over a long period of time. In the case of theultrasonic motor provided with the friction material F4 (for comparison)made of engineering plastics, the resonance frequency greatly fluctuated(6.5 kHz) according as the pressure changed (in terms of braking torquefrom 500 gf·cm to 1000 gf·cm) and the braking torque greatly changedwith time. The friction material wore the surface of the stator Therestarting of the motor became unstable after continuous running for along time.

The motors provided with one of the friction materials (A4 to D4)pertaining to the present invention generated no noise. By contrast, theultrasonic motor provided with the friction material (E4) made offluorocarbon resin alone or the friction material (F4) made ofengineering plastics became to generate noise after running for a longperiod of time.

The effects attainable by the employment of the organic polymer in thisembodiment are the improvement of starting characteristics, which is aproblem in the case where fluorocarbon polymer simplex friction materialis used, and the attainment of a larger starting torque for the samemotor pressure because of the higher friction coefficient produced bythis material.

The above-mentioned embodiments (a) to (g) indicate that the followingfunctions and effects are produced when the ultrasonic motor isconstructed such that a friction material made of at least fluorocarbonresin is present on the contact surface of at least either of the rotoror the stator.

(1) The fluorocarbon polymer has the ability to absorb vibration ofspecific frequency on account of its characteristic flexibility.Consequently, it completely prevents the ultrasonic motor from producingnoise.

(2) Since the fluorocarbon polymer works as a solid lubricant on thecontact surface and has an adhesive property, it provides smoothslipping between the stator and the rotor. Consequently, it permits theultrasonic motor to produce a stable driving force.

(3) Owing to its comparatively high heat resistance, the fluorocarbonpolymer does not suffer from seizure or melting even under the severefriction conditions produced by ultrasonic vibration. Consequently, itgreatly reduces the wear of the contact surfaces of the ultrasonic motorand permits the ultrasonic motor to maintain its performance over a longperiod of time.

(4) Since the fluorocarbon resin has a lower hardness than the metallicmaterial constituting the stator, it does not damage the contact surfaceof the stator and keeps the contact surface of the stator stable at alltimes. Therefore, the ultrasonic motor has a constant holding torque(which is a force to hold the rotor in position when the motor is atrest).

While specific embodiments of the invention have been illustrated anddescribed herein, it is realized that other modifications and changeswill occur to those skilled in the art. It is, therefore, to beunderstood that the appended claims are intended to cover allmodifications and changes as fall within the true spirit and scope ofthe invention.

We claim:
 1. In an ultrasonic motor wherein a moving body is contractedunder a pressure on a vibrating body on which a piezoelectric body isdisposed, and an ultrasonic traveling wave is produced on saidpiezoelectric body and said vibrating body by inputting high-frequencyelectric power of ultrasonic frequency to said piezoelectric body,whereby said moving body is driven by means of a wave-front of saidultrasonic traveling wave through a friction means, the improvementwherein said moving body or the contact surface of said moving body ismade of a composite plastic resin material consisting essentially of afluorocarbon resin in an amount of 100 parts by weight and awhisker-shaped powder in an amount of 2 to 100 parts by weight uniformlydistributed therein, said whisker-shaped powder being selected from thegroup consisting of potassium titanate, silicon nitride, silicon carbideand zinc oxide.
 2. The improvement according to claim 1 in which thewhisker-shaped powder is a needle-like powder having a diameter smallerthan 5 μm and an aspect ratio greater than 5.